Electronic image sensors are found in a variety of useful products, including cameras, camcorders, cell phones, medical devices, machine vision instruments, and the like. Image sensors have a characteristic dynamic range. Dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. It is desirable to have an image sensor with a high dynamic range to image scenes that generate high dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
For example in cameras there are generally two ways to adjust image exposure to achieve high dynamic range. The first is to change the shutter speed. The second is to change the size of the aperture. Both of these ways of adjusting image exposure control the quantity of light that is applied to film or to an electronic image sensor. In digital cameras, the image sensor can change the shutter speed electronically. However the use of pulsed light emitting diode (LED) light has become common, and keeping sensor integration time constant is even more important because of possible flickering effect related to short integration time. Moreover changing the lens aperture is not very practical for machine vision applications.
There are several known techniques for extending the dynamic range of image sensor pixels themselves, including the use of companding pixels, logarithmic pixels, dual conversion gain pixels, and dual photodiode pixels. However each of these approaches has drawbacks. A drawback of companding pixels is their nonlinear response, high pixel fixed pattern noise (FPN), and drop in signal-to-noise ratio (SNR) at knee points. The logarithmic pixel is also subject to high pixel FPN and nonlinearity. Dual conversion gain and dual photodiode pixels have only two predetermined, discrete responsivity values, which limit their usefulness.
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